Magnetic recording media are having to meet increasingly high requirements with respect to recording, playback and aging resistance. The binder is becoming more and more important for meeting these requirements.
Higher packing density of magnetic material in the layer is therefore desirable for improving the magnetic properties, which leads to a reduction in the binder content of the layer. Attempts have also been made to obtain an improved signal/noise ratio by the use of increasingly finely divided magnetic materials having a pronounced acicular shape. These materials are furthermore very often surface-modified to reduce aging phenomena. Because of such measures, both the distribution of the pigments during the dispersing process and the achievement of good dispersion stability are made considerably more difficult. In general, however, the magnetic layers must be very flexible and have high resilience and good tensile strength. In addition, in order to avoid drops in output level, there is an increasing need for a reduction in the coefficients of friction, an increase in the abrasion resistance and better wear properties. Moreover,, the magnetic layer must be mechanically stable, in particular at high temperatures and high atmospheric humidity.
It is known that magnetic layers which are subjected to great mechanical stresses contain polyurethane elastomers which prove advantageous as binders. Polyesterurethanes as described in DE-B 1 106 959, DE-B 2 753 694, EP-A 0 069 955 or U.S. Pat. No. 2,899,411 have proven particularly suitable.
However, these binders cannot be used in the light of the requirements and measures described above. In many cases, pigment wetting and pigment dispersing are adversely affected so that any sintered material is insufficiently broken up during the milling process or agglomeration of pigment particles is not adequately prevented, leading to poor orientability and hence to reduced packing densities. Relatively small amounts of low molecular weight dispersants are therefore added in order to facilitate the dispersing process. Although these dispersants have good deagglomeration characteristics, they do not satisfactorily stabilize the dispersion. Higher molecular weight dispersing resins are is therefore often also used as additional components. DE-A 30 26 357 and DE 31 37 293 describe, for example, the addition of polyester resins which have SO.sub.3 M groups.
However, these methods for improving the dispersing process have disadvantages. Thus, low molecular weight dispersants may be exuded under unfavorable climatic conditions, such as high temperature and/or high atmospheric humidity. This results in deposits on all parts in contact with the tape, in particular on the head, in recording or playback apparatuses, causing drops in output level. When dispersing resins are used, it is possible, on the other hand, for compatibility problems to occur in the dispersion. Often, these substances are not film formers and therefore also result in blocking. Moreover, the mechanical properties of these dispersing resins are often not adapted to the properties of the polyurethane used as the main binder. A deterioration in the mechanical properties always also means an increase in abrasion.
To improve the dispersing properties of the polyurethane binder itself, it was proposed at an early stage to incorporate polar groups into the binder. These polar groups can in principle be introduced by any component which is used in the preparation of the polyurethane. Polyesters having polar groups (cf. inter alia DE-A 28 33 845) are most frequently used. The incorporation of diols which carry additional polar groups is described in, for example, JP-A 57 092 421, German Laidopen Application DOS 3,814,536 or EP-A 193 084. The subsequent incorporation of the polar groups by nucleophilic substitution (S.sub.n) reaction at the terminal OH groups of the polyurethanes is disclosed in JP-A 57 092 422. The polyurethanes described to date and carrying polar groups have improved dispersing behavior but the improvement is still insufficient for many requirements.
Another disadvantage of all the polyurethanes described is that the required resilience is frequently obtained at the expense of insufficient hardness and a tendency to surface tack. In the prior art, appropriate polyurethanes are therefore combined with other binders.
Proposed binder combinations are, for example, mixtures of polyurethanes with phenoxy resins, with vinylidene chloride/acrylonitrile copolymers, with vinyl chloride/acrylate copolymers, with polycarbonates or with polyesters. DE-A 32 39 160 may be mentioned by way of example. Although these binder combinations lead to an improvement in the mechanical properties of the magnetic layer, the dispersing behavior of such a combination is adversely affected. Consequently, the particular properties of the magnetic materials are not satisfactorily displayed. This is manifested in a lower orientation ratio, lower residual induction and hence lower sensitivity at short and long wavelengths and a less satisfactory output level of the resulting recording media.
A possible method for increasing the hardness of the polyurethanes is to increase the concentration of urethane and urea groups. However, such measures very rapidly lead to products which are insoluble in conventional solvents, such as tetrahydrofuran (EP-A 01 43 337). According to DE-A 31 37 293, nonmagnetic particles are admixed to increase the hardness.
However, the measures described are insufficient for simultaneously meeting the increased requirements with regard to the binder system. Furthermore, a combination of the polyurethanes and other binders is often essential for achieving individual effects.
It is an object of the present invention to provide thermoplastic and resilient polyurethanes which are readily soluble in ethers and/or ketones and have excellent dispersing properties and through the use of which it is possible to produce magnetic layers having high mechanical stability and improved recording characteristics.